This invention relates generally to television systems and particularly to a digital television system that is operated in the same service area as a conventional NTSC type television system. The present terrestrial broadcast television system in the United States is identified as CCIR, system M. The frequency band is 6 MHz with a Nyquist slope in the receiver at the video carrier frequency. The audio carrier and the addition of the color subcarrier in the NTSC alters the frequency and power relationships of the system.
Years ago, the visual acuity, i.e. resolution ability of the human eye was investigated with respect to video images generated in CCIR, system M color systems. Subjectivity relationships were developed at viewing distances of approximately six times image height and were expressed in the form of a curve showing the relationship of visual sensitivity to noise. It is shown that the sensitivity of the human eye to noise decreases as the frequency is increased. For NTSC color, the relationship is altered in the area of the color subcarrier, i.e. around 3 MHz. The subjective characteristics of the eye's visual acuity in combination with the frequency characteristics of an NTSC television receiver are combined herein to generate a random, white, noise subjective weighting baseband curve for a CCIR, system M color receiver. This characteristic is referred to herein as NTSC receiver sensitivity. It should be borne in mind, however, that the concept includes the aspects of visual acuity of the human eye.
Proposals for a new digital high definition television system are being considered for adoption in the United States. One such system is known as a digital, spectrum-compatible, high definition television (DSC-HDTV) which is jointly sponsored by Zenith Electronics Corporation and AT&T. This high definition digital transmission system will be referred to herein as ATV, but it should be understood to encompass any digital television transmission system. Digital transmission systems have power distribution curves that are noise like, i.e. they uniformally extend over the frequency spectrum in the manner of white noise.
A major obstacle to the selection of a new television system for the United States is the existence of a vast number of NTSC receivers and corresponding transmitters which must remain operable for a significant number of years while the new system is introduced. Thus many, if not all, service areas will be subject to both ATV television signals and NTSC television signals. Clearly, measures must be taken to minimize the adverse influences of one type of signal on receivers of the other type. The present invention is directed to tailoring the ATV signal power to complement NTSC receiver sensitivity such that a higher power ATV signal is transmitted in less sensitive areas of the NTSC receiver sensitivity spectrum and a lower power ATV signal is sent in the more sensitive NTSC frequency areas. This promotes not only reduced co-channel interference from the ATV signal into the NTSC receiver, but will also enable a higher power ATV signal without detrimental co-channel interference.